Ocean carbonate system variability in the North Atlantic Subpolar surface water (1993-2017)

Coraline Leseurre (1) , Claire Lo Monaco (2) , Gilles Reverdin (3) , Nicolas Metzl (1) , Jonathan Fin (2) , Sólveig R. Ólafsdóttir (4) , Virginie Racapé (5)

1 LOCEAN - Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques
2 CYBIOM - Cycles biogéochimiques marins : processus et perturbations
3 PROTEO - Processus et interactions de fine échelle océanique
4 MFRI - Marine and Freshwater Research Institute
5 LOPS - Laboratoire d'Océanographie Physique et Spatiale

The North Atlantic is one of the major ocean sinks for natural and anthropogenic atmospheric CO 2. Given the variability of the circulation, convective processes or warming-cooling recognized in the high latitudes in this region , a better understanding of the CO 2 sink temporal variability and associated acidification needs a close inspection of seasonal, interannual to multidecadal observations. In this study, we investigate the evolution of CO 2 uptake and ocean acidification in the North Atlantic Subpolar Gyre (50-64 • N) using repeated observations collected over the last 3 decades in the framework of the long-term monitoring program SURATLANT (SURveillance de l'ATLANTique). Over the full period (1993-2017) pH decreases (−0.0017 yr −1) and fugacity of CO 2 (f CO 2) increases (+1.70 µatm yr −1). The trend of f CO 2 in surface water is slightly less than the atmospheric rate (+1.96 µatm yr −1). This is mainly due to dissolved inorganic carbon (DIC) increase associated with the anthropogenic signal. However, over shorter periods (4-10 years) and depending on the season, we detect significant variability investigated in more detail in this study. Data obtained between 1993 and 1997 suggest a rapid increase in f CO 2 in summer (up to +14 µatm yr −1) that was driven by a significant warming and an increase in DIC for a short period. Similar f CO 2 trends are observed between 2001 and 2007 during both summer and winter, but, without significant warming detected, these trends are mainly explained by an increase in DIC and a decrease in alkalinity. This also leads to a pH decrease but with contrasting trends depending on the region and season (between −0.006 and −0.013 yr −1). Conversely , data obtained during the last decade (2008-2017) in summer show a cooling of surface waters and an increase in alkalinity, leading to a strong decrease in surface f CO 2 (between −4.4 and −2.3 µatm yr −1 ; i.e., the ocean CO 2 sink increases). Surprisingly, during summer, pH increases up to +0.0052 yr −1 in the southern subpolar gyre. Overall, our results show that, in addition to the accumulation of anthro-pogenic CO 2 , the temporal changes in the uptake of CO 2 and ocean acidification in the North Atlantic Subpolar Gyre present significant multiannual variability, not clearly directly associated with the North Atlantic Oscillation (NAO). With such variability it is uncertain to predict the near-future evolution of air-sea CO 2 fluxes and pH in this region. Thus, it is highly recommended to maintain long-term observations to monitor these properties in the next decade.

Domaines: Océan, Atmosphère; Milieux et Changements globaux

Coraline Leseurre, Claire Lo Monaco, Gilles Reverdin, Nicolas Metzl, Jonathan Fin, et al.. Ocean carbonate system variability in the North Atlantic Subpolar surface water (1993-2017). Biogeosciences, 2020, 17 (9), pp.2553 - 2577. ⟨10.5194/bg-17-2553-2020⟩. ⟨hal-02573889⟩